docs(brand): rename to codewhale across READMEs and docs

Sweep brand mentions of `DeepSeek TUI` / `deepseek-tui` / bare
`deepseek` (the dispatcher binary) across all user-facing docs to
the new `codewhale` brand. The DeepSeek **provider** integration is
left untouched throughout: env vars (`DEEPSEEK_*`), model IDs
(`deepseek-v4-pro`, `deepseek-v4-flash`, `deepseek-chat`,
`deepseek-reasoner`), the `api.deepseek.com` host, the
`~/.deepseek/` config dir, and the `--provider deepseek` argument
value all keep the legacy spelling.

Anti-scope items deliberately left as the legacy `deepseek-tui`:

- Homebrew tap and formula (`Hmbown/homebrew-deepseek-tui`,
  `brew install deepseek-tui`, `scoop install deepseek-tui`). The
  tap rename ships separately.
- Docker image (`ghcr.io/hmbown/deepseek-tui`). Image-tag rename
  ships separately.
- CNB mirror namespace (`cnb.cool/deepseek-tui.com/DeepSeek-TUI`).
  Third-party hosted path.
- Security contact email (`security@deepseek-tui.com`).
- GitHub repo URL (`Hmbown/DeepSeek-TUI`).

New artifact:

- `docs/REBRAND.md` documents what changed, what didn't, the
  deprecation window, and migration commands for npm / Cargo /
  Homebrew / manual installs.

CHANGELOG entries:

- Root `CHANGELOG.md` and `crates/tui/CHANGELOG.md` both gain a
  new `[Unreleased]` section describing the rename and the one-
  release deprecation window. Historical entries are untouched.

Issue templates:

- `.github/ISSUE_TEMPLATE/bug_report.md` and `feature_request.md`
  refer to "codewhale" / `codewhale --version` instead of the old
  brand name in their environment fields.

The rebrand sweep was driven by a perl script with bulk patterns
(`deepseek-tui` -> `codewhale-tui`, `DeepSeek TUI` -> `codewhale`,
bare `deepseek` -> `codewhale` with provider/model/host/env-var/
config-path negative lookbehind/lookahead) followed by targeted
reverts for the anti-scope items above. Output was visually
reviewed file-by-file before committing.

Verified:

- `cargo check --workspace --all-targets --locked` — pass.
- `cargo test --workspace --all-features --locked` — pass (no
  test source touched here; suite stayed green to confirm no
  doc-from-string assertions broke).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/ISSUE_TEMPLATE/bug_report.md

@@ -27,7 +27,7 @@ labels: bug
 ## Environment
 
 - OS:
-- DeepSeek CLI version:
+- codewhale version:
 - Install method:
 - Model/provider:
 - Terminal app:
@@ -35,7 +35,7 @@ labels: bug
 
 <!-- Hints:
 OS: Windows 11 / Ubuntu 22.04 / macOS 14
-DeepSeek CLI version: run `deepseek --version`
+codewhale version: run `codewhale --version`
 Install method: cargo install / release binary / source build
 Model/provider: deepseek-v4-pro / DeepSeek, or qwen2.5-coder / Ollama
 Terminal app: iTerm2 / Windows Terminal / GNOME Terminal / VS Code terminal

.github/ISSUE_TEMPLATE/feature_request.md

@@ -10,7 +10,7 @@ labels: enhancement
 
 ## Proposed solution
 
-<!-- What should DeepSeek TUI do differently? Include commands, UI behavior, or config shape if relevant. -->
+<!-- What should codewhale do differently? Include commands, UI behavior, or config shape if relevant. -->
 
 ## Use case